1. Field of the Invention
The present invention relates to a ceramic package type electronic part. More particularly, the present invention relates to a ceramic package type electronic part which is high in connection strength to an electrode.
2. Description of the Related Art
IC devices and other electronic parts have been improved year by year. Operation of high accuracy is required of a high graded electronic device, for example, an IC device and an SAW (Surface Acoustic Wave Filter) resonator. Thus, in many cases, it is used in a condition that it is mounted in a hermetically sealed package. Such a package is required to protect the electronic device against the high density of a wiring circuit, the thermal generation thereof, the change of temperature and moisture, dust and other ambient environments. Such a hermetically sealed package has been developed more and more in recent years.
FIG. 1 shows a ceramic package type electronic part 190 created by hermetically mounting the electronic device in a so-called ceramic package, in such a hermetically sealed package. In FIG. 1, a cover 193 is illustrated at a top of FIG. 1, an electronic device (for example, a piezoelectric device, such as a quartz oscillator) 192 is illustrated thereunder, and a base 191 is further illustrated thereunder. In this ceramic package type electronic part 190, an inner electrode 197 positioned on the base 191 and the quartz oscillator 192 are electrically connected to each other. The quartz oscillator 192 hermetically sealed by guiding a conductive line to an outer connection portion (outer electrode) 125 linked to the inner electrode 197 is adapted to serve as a part of an outer electronic circuit.
FIG. 2 is a side section view showing the ceramic package type electronic part 190 of FIG. 1.
In the ceramic package type electronic part 190, the quartz oscillator 192 is hermetically enclosed and positioned within a ceramic case 122. At right and left ends of the quartz oscillator 192, it is connected through conductive adhesives 124 to the inner electrode 197. The inner electrode 197 is integrally extended up to the outer connection portion 125, and connected through solder or the like to, for example, a wiring on a printed circuit board on which this ceramic package type electronic part 190 is mounted, outside the ceramic case 122.
The side on which the quartz oscillator 192 of this ceramic package is positioned, namely, the portion between the base 191 and the hermetically enclosing cover 193 thereon is hermetically sealed and bonded with a glass 126. The reason why the portion is hermetically sealed and adhered with the glass 126 is as follows. That is, a very precise operation is required of the piece of the quartz oscillator 192. Thus, the change of the inner ambient of this ceramic case 122 has influence on the operation. Hence, the portion between the base 191 and the cover 193 is typically sealed with the glass 126, from the viewpoint of the hermetic sealing, the reliability thereof and the like.
FIG. 3 is a side section view showing another ceramic package or another ceramic package type electronic part. As shown in FIG. 3, a quartz oscillator 202 is connected through a conductive substrate 205 to an inner electrode 207 located on a left side of a base 201, and supported in a cantilever state. This inner electrode 207 is linked to an outer electrode 208 through a conductive line 206 wired on ceramic. Moreover, after the quartz oscillator 202 serving as the electronic device is fixed on the base 201, an opening is enclosed by a cover 209, and the quartz oscillator 202 is hermetically sealed. Usually, this cover 209 hermetically encloses the base 201 made of ceramic material by using glass and the like.
A method of typically manufacturing such a ceramic package or a ceramic package type electronic part will be described below. In general, such a ceramic package or a ceramic package type electronic part is manufactured by using a green sheet method. In the green sheet method, element on which conductor paste is printed is laminated on alumina green sheet created by a doctor blade method, and then the laminated element is collectively baked. The green sheet method is a method of manufacturing a circuit substrate having a high reliability for various type of electronic devices, quartz oscillators, piezoelectric actuators, SAW oscillators, ICs, LSIs and VLSIs or other devices with high reliability.
The ceramic body, such as a multiple-layer circuit substrate, a ceramic package or the like, made by this method has the following features.
Firstly, the easy lamination of a number of sheets each having hyperfine wiring enables the high density wiring.
Secondly, the operation of baking and forming an Insulation substrate and a conductor at the same time makes the integration thereof perfect and also makes the reliability thereof high.
A basic process of manufacturing the multiple-layer circuit substrate and the like, for example, the ceramic package by using the green sheet method will be described below with reference to FIGS. 4A to 4I and FIGS. 5A to 5E.
As shown in FIG. 4A, alumina of raw material powder, flux, binder, plasticizer and solvent are sufficiently mixed with each other in a ball mill and made into slurry. As shown in FIGS. 4B and 4C, element in which this slurry is extended and dried on a carrier tape by using a blade is referred to as a green sheet. This method is referred to as the doctor blade method, and this is typically used. This green sheet has a thickness between about 0.1 and 1.0 mm. The thickness can be adjusted as necessary (FIG. 4D).
The conductive paste created in the state of metal powder having a high melting point on this green sheet is screen printed (FIG. 4E). As a method of creating the multiple-layer in the green sheet method, there are three methods: a sheet lamination method; a print multiple-layer method and a method of jointly using both the methods. However, the frequently used method is the sheet lamination method.
According to the sheet lamination method, a hole is made in the green sheet by using a die or a micro drill. The conductive paste is filled into the hole. A plurality of sheets on which patterns are printed are laminated (FIG. 4F). Then, they are baked in reduction ambience (FIG. 4H). Accordingly, the ceramic body or the ceramic package body is manufactured (FIG. 4I).
Then, such a ceramic body is machined to a form of case. An electronic device whose function is deteriorated by environment is mounted in the case. Then, it is hermetically sealed and put to practical use. In this case, a so-called box-shaped portion of this ceramic case is referred to as a base, and an element placed thereon is referred to as a cover.
The above-mentioned sheet lamination method is a very excellent method. The schematic specification is, for example, as follows.
Wiring material is tungsten system material, molybdenum system material or other materials. About 0.08 mm is possible as the minimum line width, about 0.1 mm is possible as the minimum line interval, about 0.1 mm is possible as the minimum through hole diameter, and about 0.25 mm is possible as the minimum through hole pitch. Also, alumina between about 90 and 94% is used as the material of the conventionally used raw material powder. Its thermal expansion coefficient is 75.times.10.sup.-7 /.degree.C., its dielectric constant is 8.5 and its specific resistance is about 10.times.10.sup.14 .OMEGA./cm. The number of about 45 layers or more is possible as the maximum number of laminations of green sheets laminated as mentioned above.
Here, the above-mentioned wiring material connects the electric device mounted inside this ceramic case to the wiring positioned on, for example, a printed circuit board and the like mounted outside this ceramic case. There are an inner electrode and an outer connection portion in the wiring material. The inner electrode is a portion wired inside the ceramic case. The outer connection portion is wired outside the ceramic case, and electrically connected to the printed circuit board and the like. The inner electrode and the outer connection portion can be connected to each other through a wiring of a predetermined interval by using the tungsten or the molybdenum. Or, the inside of the through hole and the like are formed with conductive material through which the outer connection portion and the inner electrode can be electrically connected to each other.
A process of integrating a plurality of green sheet into a single unit will be described below. The plurality of green sheets are made into the single unit by molding after the lamination. The most recommended integrating method is a low temperature isotropic consolidation molding method, namely, a so-called CIP method. This CIP method is explained. At first, laminated green sheets serving as raw material are put into a rubber bag. They are put into pressure transfer solution (pure viscous liquid) accommodated in a compression vessel. Under this condition, the transfer solution is compressed. Accordingly, raw material powder in the bag is consolidated in isotropy by pascal pressure generated in the transfer solution. Since it is pressed in isotropy from all quarters by the pascal pressure, further high isotropic compression, high dense compression and uniform compression can be generated as compared with a case of single axis consolidation. Thus, it is possible to obtain high density molding which is superior in formation.
This method will be described below with reference to FIGS. 5A to 5E. At first, green sheets 110 in which ceramic sheets are punched to target shapes are prepared (FIG. 5A). The green sheets 110 are machined to green sheets 111, 112 on which patterns are printed (FIG. 5B). These green sheets are piled up to create a lamination body 113 (FIG. 5C). This lamination body 113 is put into a vinyl bag 114 or a rubber bag to then pack it (FIG. 5D). The packed vinyl bag 114 or rubber bag into which the lamination body 113 of the green sheets is put is made vacuous so that the vinyl bag 114 or the rubber bag is adhered to the lamination body 113 of the green sheets (FIG. 5E). Then, isotropic press is applied thereto to generate a ceramic body 115 (FIG. 5E).
In addition, there is a high temperature isotropic consolidation molding method, namely, a so-called HIP method, other than the low temperature isotropic consolidation molding method. However, this method employs the similar principle. The reason why such a low temperature isotropic consolidation molding method is carried out is as follows. That is, the isotropic press causes the binders contained in the green sheets to be softened. Then, the portions between the green sheets are adhered to each other with the softened binders. Finally, the ceramic body 115, namely, a ceramic package or an intermediate body is completed.
As mentioned above, the ceramic package or the ceramic package type electronic part is completed as shown in FIG. 1. The ceramic package or the ceramic package type electronic part is finally placed on a mother board, namely, a circuit board to then create a circuit having one function as a whole.
As can be understood from the above-mentioned description, a process of especially controlling a surface roughness of the green sheet is not included in the above-mentioned manufacturing method. A surface of the green sheet typically has a surface roughness between about 1 and 50 microns. On the other hand, the above-mentioned wiring is wired by various methods. However, the material thereof is made of metal, and a surface roughness thereof is relatively superior to that of the surface of the green sheet. That is, as to the surface roughness, the surface of the wiring, namely, the inner electrode is smooth, and the surfaces of the green sheet, namely, the bottom side and the inner wall of the ceramic case are rough.
As shown in FIG. 2, the conductive adhesive 124 is used to connect the quartz oscillator 192 and the inner electrode 197 to each other. In this conductive adhesive 124, a number of metal powders each having a small particle diameter are enclosed in organic material. The conductive adhesive 124 uses the metal powders to then ensure conductive paths so that the quartz oscillator 192 and the inner electrode 197 are electrically connected to each other. By the way, the electronic device represented by such an enclosed quartz oscillator must be surely fixed to the inner electrode 197, from the viewpoint of the insurance of its operational accuracy and the like. Of course, the electronic device does not function if it is removed from the inner electrode 197. Moreover, if crack is induced in a part of the inner electrode 197, or if the resistance of the connection portion between the inner electrode 197 and the quartz oscillator 192 is changed by any impact, it has influence on the operational accuracy to thereby result in a problem.
Here, a problem shown in FIG. 6 may be induced. As shown in FIG. 6, if impact is applied from external portion, this results in a problem that a conductive adhesive 134 for connecting between an electronic device 131 (in this example, a quartz oscillator) enclosed within a ceramic case 132 and an inner electrode 133 is stripped from the inner electrode 133, or a crack is induced between the inner electrode 133 and the electronic device 131. The reasons why these problems are induced are as follows. Firstly, since the quartz oscillator 131 itself is enclosed within the ceramic case 132 only with the conductive adhesive 134, the strong force is applied to that portion. Secondly, the adhesive strength between the inner electrode 133 and the conductive adhesive 134 is weak.
Thus, in order to solve the above-mentioned problems, it is enough to employ the adhesive whose adhesive strength is stronger. However, a rate of organic solvent occupying the conductive adhesive must be increased to make the adhesive strength stronger. As a result, a rate of the metal particles which ensure the conductivity and are small in particle diameter becomes small, which results in problems that if the resistance thereof is increased, the reliability of the ceramic package type electronic part is poor, or if the resistance thereof is excessively increased, trouble is incurred in a circuit design.
Hence, from the above-mentioned standpoints, it is desirable that the conductive adhesive contains the metal particles with minor particle diameters as much as possible to thereby reduce the rate of the organic solvent occupying the conductive adhesive. However, this method can not improve the adhesive strength between the conductive adhesive and the inner electrode.
As mentioned above, this type of ceramic package or the ceramic package type electronic part in which the electronic device is enclosed therein is electrically mechanically connected and fixed to the electronic circuit on the mother board by soldering, when it is placed on the mother board (circuit board). Usually, this connection is carried out by using solder. In this case, as the soldering manner, a method referred to as a re-flow solder is mainly used especially in recent years.
The re-flow solder is simply described. Solder material in a form of cream paste is mounted on a printed board by print. This type of ceramic package or ceramic package type electronic part is placed on this solder material, and passed through a high temperature furnace. Accordingly, the cream solder is fused. After that, it is cooled. Then, the ceramic package or the ceramic package type electronic part is electrically mechanically connected on the mother board.
By the way, there is a method referred to as a dip solder, as another conventional method. The dip solder is a method of dipping electronic devices mechanically fixed on a circuit board into molten solder solution and then removing them. Accordingly, the solder easily adapted to the metal exposure portions on the circuit board and in the ceramic package stays thereon. Then, it is cooled to accordingly carry out the electric mechanic connection similar to the above-mentioned case.
There are additionally various methods. In a general, various electronic parts placed on the mother board, for example, the ceramic package and the ceramic package type electronic part in the present invention are designed so as to create one electronic circuit as a whole through the solder connection.
In the ceramic package or the ceramic package type electronic part in the present invention, the electronic devices to be accommodated therein are required to be placed within it. For this reason, this portion also needs the electrical connection and the mechanical connection. However, as mentioned above, the very high accuracies are required of the electronic devices to be hermetically enclosed and placed within it. They are very weak against heat, various gases, dusts and the like. Thus, in many cases, it is difficult to directly solder this electronic device. Hence, a method other than the soldering, for example, a conductive organic adhesive is used if, for example, the electronic device such as the quartz oscillator is mounted within the ceramic package. In this case, the solder is still used for the connection between the circuit board and the outer electrode mounted outside the ceramic package.
However, this conductive organic adhesive has characteristic different from the solder used when the ceramic package or the ceramic package type electronic part is mounted on the circuit board (mother board). As a result, if the outer electrode guided from the ceramic package to the external portion and the inner electrode within the ceramic package are made of the material identical to each other, this leads to the trouble that, for example, a certain material is superior in adaptation to the conductive adhesive and inferior in adaptation to the solder. On the contrary, a certain material is superior in adaptation to the solder and inferior in adaptation to the conductive adhesive.
Such inferiority in the adaptation between the metal and the conductive adhesive or the solder is not in the degree to which the electronic device can not be always created. Thus, conventionally, the structure of the electrode of the ceramic package is determined by sacrificing the adhesive strength in any one of the outer electrode and the Inner electrode.
For example, if the electronic device hermetically enclosed within the ceramic package must be strongly fixed against vibration from external portion and the like, the connection strength between the inner electrode and the electronic device within the ceramic package is emphasized by sacrificing the connection through the solder between the circuit board and the outer electrode.
However, the reliability of the electronic device within the ceramic package as well as the circuit board in which such electronic devices are mounted has been separately required at high levels, in recent years. The policy of sacrificing any one of them has not been allowed from those circumstances.
A thesis entitled "Tendency of Ceramic Package for Quartz Device" written by Ono is noted in "Quartz Device" 18 No.1, 1996, pages 36-38, published in Japanese Quartz Device Industry Association. This thesis announces that an alumina multiple-layer ceramic package can satisfy a condition of hermetic reliability and an insurance of space for accommodating a piezoelectric device, in a surface mount operation of a quartz device. It is noted therein that a surface mount type can improve the connection reliability since solder is protuberant on metallization of a side groove at a time of solder re-flow. As a process of manufacturing the ceramic package, it is noted therein that a product is completed by carrying out break-out/inspection after plating/brazing, after firing, after scoring after laminating, after pattern printing, after via printing, after punching, after blank sheet and after tape making.